[btuck812]

I am using CFX to model a transient system involving a moving fluid with a small number of dissolved particles entering via an inlet and leaving via an outlet, adjacent to a hydrogel. Both the hydrogel and the moving fluid are water based so the dissolved particles are able to diffuse into the hydrogel at a standard diffusion coefficient. However, the hydrogel is solid and therefore has no flow.

I have worked out how to allow diffusing particles into the hydrogel whilst maintaining the zero-flow situation, by separating the fluid and hydrogel domains with a no-slip wall, and implementing the particles as an additional variable with interface flux, which works nicely. However, I also need the particles to react (based on anaerobic respiration, as the hydrogel will contain bacteria) and additional variables don't seem to have this ability. I don't know how to do this. Is there a way to use multicomponent flow for this purpose? Can I permanently restrict fluid velocity in the hydrogel domain?

Any advice would be very welcome, I recognise this probably isn't a typical problem to solve in this software and if, for example, Fluent would work better, please do let me know.
I can provide images if this would help

Thank you all

[ghorrocks]

If the stuff reacts and you want to keep track of the reaction inputs and outputs then you need to use a multicomponent mixture and chemistry. This has similar options to the additional variable you are already using so you should be able to transfer your existing additional variable model to a multicomponent mixture.

Also note that you may be able to use your existing additional variable model to include some simple chemistry using source terms. I would only implement simple chemistry with this approach, for instance with low heat of reaction for instance, as this approach does not include the numerical changes required to handle more difficult reactions.

[btuck812]

Thank you for your response. I am able to set up the model as a multicomponent mixture, but I can't work out how to keep the flow in the liquid channel from affecting flow in the hydrogel which cannot have any flow. I have tried using a thin porous domain between the two, but flow in the channel still causes flow in the hydrogel which is a major problem. Is there a way to segregate flows whilst allowing diffusion between in multicomponent modelling?

[ghorrocks]

Can you make the hydrogel a solid with a diffusion coefficient?

[btuck812]

I can't find any option to give a solid domain a diffusion coefficient, could you please give me a bit more detail?
Thanks

[ghorrocks]

Define a solid domain and an additional variable. On the for the solid domain under the Solid Model tab you can set a diffusivity.